Electrosurgical generators are used by surgeons to cut and coagulate tissue of a patient. High frequency electrical power is produced by the electrosurgical generator and applied to the surgical site by an electrosurgical tool. Monopolar and bipolar configurations are common in electrosurgical procedures.
Electrosurgical generators are typically comprised of power supply circuits, front panel interface circuits, and RF output stage circuits. Many electrical designs for electrosurgical generators are known in the field. In certain electrosurgical generator designs, the RF output stage can be adjusted to control the RMS output power. The methods of controlling the RF output stage may comprise changing the duty cycle, or changing the amplitude of the driving signal to the RF output stage. The method of controlling the RF output stage is described herein as changing an input to the RF output stage.
Electrosurgical techniques have been used to seal small diameter blood vessels and vascular bundles. Another application of electrosurgical energy is tissue welding. In this application, two layers of tissue are grasped and clamped together while electrosurgical power is applied. The two layers are thereby welded together. Tissue welding is similar to vessel sealing, except that a vessel or duct is not necessarily sealed in this process. For example, tissue welding may be used instead of staples for surgical anastomosis. Electrosurgical power has a desiccating effect on tissue during tissue welding or vessel sealing. As used herein, the term "electrosurgical desiccation" is meant to encompass any tissue desiccation procedure, including standard electrosurgical coagulation, desiccation, vessel sealing, and tissue welding.
One of the problems associated with electrosurgical desiccation is undesirable tissue damage due to thermal effects. The tissue at the operative site is heated by the electrosurgical current. Healthy tissue adjacent to the operative site can become thermally damaged if too much heat is allowed to build up at the operative site. The heat may conduct to the adjacent tissue and cause a large region of tissue necrosis. This is known as thermal spread. The problem of thermal spread becomes important when electrosurgical tools are used in close proximity to delicate anatomical structures. Therefore, an electrosurgical generator that reduced the possibility of thermal spread would offer a better opportunity for a successful surgical outcome.
Another problem that is associated with electrosurgical desiccation is a buildup of eschar on the surgical tool. Eschar is a deposit on an electrosurgical tool that is created from tissue that is desiccated and then charred by heat. The surgical tools will often lose effectiveness when they are coated with eschar. The buildup of eschar could be reduced when less heat is developed at the operative site.
Practitioners have known that a measurement of electrical impedance of tissue is a good indication of the state of desiccation of the tissue. Several commercially available electrosurgical generators can automatically terminate output power based on a measurement of impedance. Several methods for determining the optimal point of desiccation are known in the field. One method sets a threshold impedance, and terminates power once the measured impedance of the tissue crosses the threshold. Another method terminates power based on dynamic variations in the impedance.
A discussion of the dynamic variations of impedance of tissue can be found in the article, Vallfors and Bergdahl, "Automatically Controlled Bipolar Electrocoagulation," Neurosurgical Review, 7:2-3, pp. 187-190, 1984. FIG. 2 in the Vallfors article shows impedance as a function of time during heating of tissue. Vallfors reports that the impedance value of tissue proved to be close to minimal at the moment of coagulation. Based on this observation, Vallfors suggests a micro-computer technique for monitoring the minimum impedance and subsequently terminating output power to avoid charring the tissue.
A second article by Bergdahl and Vallfors, "Studies on Coagulation and the Development of an Automatic Computerized Bipolar Coagulator," Journal of Neurosurgery, 75:1, 148-151, July 1991, discusses the impedance behavior of tissue and its application to electrosurgical vessel sealing. The Bergdahl article reported that the impedance had a minimum value at the moment of coagulation. The Bergdahl article also reported that it was not possible to coagulate safely arteries with a diameter larger than 2 to 2.5 millimeters. The present invention helps to overcome this limitation by enabling electrosurgical vessel sealing of larger diameter vessels.
U.S. Pat. No. 5,540,684 discloses a method and apparatus for electrosurgically treating tissue in a manner similar to the disclosures of Vallfors and Bergdahl. The '684 patent addresses the problem associated with turning off the RF output automatically after the tissue impedance has reached a minimum value. A storage device records maximum and minimum impedance values, and an algorithm computes an optimal time for terminating output power.
U.S. Pat. No. 4,191,188 discloses a variable crest factor electrosurgical generator. The crest factor is disclosed to be associated with the coagulation effectiveness of the electrosurgical waveform.
U.S. Pat. No. 5,472,443 discloses the variation of tissue impedance with temperature. The impedance of tissue is shown to fill, and then subsequently rise as the temperature is increased. The '443 patent shows a relatively lower temperature region (Region A in FIG. 2) where salts, contained within the body fluids, are believed to dissociate, thereby decreasing the electrical impedance. The relatively next higher temperature region (Region B) is where the water in the tissues boils away, causing the impedance to rise. The relatively highest region (Region C) is where the tissue becomes charred, resulting in a slight lowering of impedance.
It would be desirable to have an electrosurgical generator that produced a clinically effective output and, in addition, reduced the amount of heat and thermal spread at the operative site. It would also be desirable to have an electrosurgical generator that produced a better quality seal for vessel sealing and tissue welding operations. It would also be designable to have an electrosurgical generator that desiccated tissue by applying a minimal amount of electrosurgical energy.